Multi-layer printed circuit boards typically include, in cross-section, a copper outer layer, followed by an epoxy-glass resin layer, a copper inner layer, another epoxy-glass resin layer, and an outer copper layer. The number of alternating copper inner layers and epoxy-glass resin layers can vary. In some printed circuit boards, the copper inner layer may not be used. In other printed circuit boards, the dielectric layer may be made from a material other than epoxy-glass resin.
Multi-layer printed circuit boards are made by a multi-stage process. Once the circuit pattern is etched onto each copper inner layer, the inner layer or layers are sandwiched between the epoxy layers and the copper outer layers, and heat and pressure are then applied to cure the epoxy and bond the layers together. A large number of small through-holes are then drilled through the cross-section of the multi-layer printed circuit board by a high speed drilling tool, in preparation for forming electrical contacts between the top and bottom outer copper layers and the inner copper layers of the printed circuit board. Usually, the contacts are made by first plating an electrically conductive metal, such as electroless copper, to the surface of the through-hole. The wall of the through-hole is then further plated with electrolytic copper to complete the contact between the copper layers of the printed circuit board.
It is not uncommon to drill thousands of through-holes in a single printed circuit board. As a result, abrasion to the drill bit is a problem because of the high drilling speed and the large amount of heat generated, especially because of drilling through the metal layers and the abrasive epoxy-glass layers. In conventional drilling operations, the drilling tool can easily reach temperatures in the neighborhood of 500.degrees. F. to 700.degrees. F.
Such drilling temperatures encountered during the drilling of through-holes in the epoxy-glass and copper layers often causes the epoxy to smear over the copper inner layers. The dielectric material also can smear onto the conductive layers when drilling panels having insulating layers made from other materials. If the through-holes are left untreated (i.e., if the smeared dielectric material is not completely removed) prior to copper plating the walls of the through-holes, a dielectric barrier can be left between the conductive path of the copper plated through-holes and the copper conductive layers. This interferes with the electrical connection and results in unreliable electrical contacts between the copper inner and outer layers (or between the conductive outer layers if an inner layer is not used). The high drilling temperatures also shortens the useful life of the drill bit.
Another problem often encountered during the high speed drilling of printed circuit boards is the creation of a burr on the periphery of the through-hole where the drill exits the copper-clad outer surface. Such a burr may form a dam-like condition during plating of the through-hole which can entrap contaminants or air. Such entrapped contaminants or air may later result in cracking of the through-hole plating and a resultant electrical failure. Such burrs thus must be removed, either by a separate sanding operation or prevented by the use of a suitable backup board. Suitable backup boards, for example are disclosed in Block U.S. Pat. Nos. 3,700,341 and 4,269,549.
Yet another problem is countered when the drilling tool enters the printed circuit board. The high speed drill bit upon encountering the surface of the printed circuit board tends to create burrs at the entry point of the through-hole. Conventionally, the problem of burrs at the entry point of the through-hole is reduced by the use of an appropriate entry material. Typically, the entry material used is a thin metal foil, such as aluminum.
The prior art methods of drilling printed circuit boards fail to adequately address the three identified problems in an integrated fashion. For example, Hatch et al. U.S. Pat. Nos. 4,781,495 and 4,929,370 disclose a drilling method in which a lubricating sheet is placed between each printed board in a stack of printed circuit boards prior to drilling through-holes in the printed circuit boars. Each sheet consists of a porous, pulp-based paper which is saturated with a water soluble dry lubricant. While the lubricating sheet provides lubrication for the drilling tool during the drilling process, the porous, pulp-based paper does not contribute to the elimination or reduction of burrs. Moreover, because the outer surface of the lubricating sheet is coated with a layer of the lubricant, the lubricant directly contacts the surface of the printed circuit board and consequently contaminates the printed circuit board so that a subsequent cleaning step is required.
Another prior art entry material consists of a thin aluminum foil with a lubricating film coated on one surface. The entry material is placed on the top surface of the stack of printed circuit boards to be drilled with the aluminum foil in contact with the top surface of the stack of printed circuit boards. While such an entry material provides lubrication to the drilling tools, the thin aluminum foil lacks dimensional stability in that it bows and twists. Such bowing and twisting results in lost of contact between the top surface of the printed circuit board and the aluminum foil. Without such contact, the entry material cannot perform the function of reducing burring.